One example of irony in A Separate Peace is when Finny fell from the tree. He fell because his friend Gene bounced the limb. So he was hurt/betrayed by the person who he trusted more than anyone else. Also, the scene where Finny is about to fall was ironic because it showed him awkward and almost clumsy when he is usually considered the best athlete in the school. Now Finny, as a result of the fall, will never be able regain his former glory as an athlete.
There was also an example of irony in the Latin inscription over the door at the school. The quote states that boys are made into men at Devon School. Traditionally the reader would picture their growth taking place through their education. However, Finny grew up and became disillusioned as a result of his fall and the loss of his athleticism. Gene grew up because of the guilt he experienced over being the cause of Finny’s injury and, eventually, the second break in his leg which led to his death.
Irony can also be seen in Ch 4 when the reader sees the way that Gene misunderstands Finny’s intentions. He keeps thinking that there is something dark about Finny. He thinks that Finny sees them as being in competition with one another and that Finny wants to keep him (Gene) from success. The fact that this is completely untrue is also ironic.
In Ch 8, Finny is helping Gene train for the 1944 Olympics. Finny is trying to live through Gene by making himself the kind of athlete he used to be. Gene works hard and improves his running time, but he is losing himself in the process and becoming more like Finny. Finny makes a comment about Gene learning more about himself through exercise. Gene has learned something, but not what Finny thinks. The irony is that he has learned how easy it is for him to change himself into what Finny wants him to be.
Later in the book (Ch 10), there is a conversation between the boys discussing Finny’s strength. Finny insists that he is doing better and Gene agrees with him. This is ironic given what happens at the end of the chapter.
One definition of Irony is when the unexpected happens; but not only that, it's also when someone does their best to avoid a situation and gets involved anyway. Another definition might be when things don't turn out the way we expect they would. In Leper's case, he was a loner for the first 9 chapters of the novel and then signs up for the army! That was a very unexpected decision because he had never shown any interest in the army and was a character who would most likely wait to be drafted before entering the war. But the irony doesn't end there. Leper goes into the army hoping to get into a special ski patrol because he likes skiing; but, he can't get past basic training for psychological reasons. Hence, the Leper gets a shot of reality opposite of what he expected, too. Another example of irony is how Gene tries to avoid publicly facing his responsibility for the tree episode by confessing to Finny. Unexpectedly, Gene is brought before a student court no matter how hard he tried to keep it a private matter between himself and Finny. Yet, the most unexpected moment for me was when Finny died in the end. He was getting better and at least going to walk again (if not ever being able to do sports again). And the one who loved life and wanted to be a part of the war and everything involved in physical activity gets crippled and later dies. It's not fair, but irony usually never is.